The script above appears to work just fine if you use OmniGraffle 4. Save the script as an application, then drag your sitemap file to that application on the desktop and OmniGraffle will draw your map. You can download a copy of OmniGraffle 4 from the Omni website if you don't have it, and either use it in demo mode or ask Omni to generate you a key if you've got an OmniGraffle 5 license already.
If the issue is that you need a tool to actually generate the sitemap file, I can't really offer you any assistance there without doing the same research that you could do :-) For my test, I just copied and pasted the one shown at
http://www.freesitemapgenerator.com/...e-sitemap.html